Why do we have to reckon ourselves as sinners?
Scripture makes plain that we must reckon ourselves as sinners because that is exactly where God finds us - while we were yet sinners. Acknowledging that true condition is necessary, for justification comes through faith in Christ's shed blood, not by pretending we are righteous.
Les teaches from the verse that while we were yet sinners, just exactly where God finds us is what we have to reckon ourselves to be. He connects this to being justified by His blood, not by drinking literal blood, but by faith in His shed blood applied to our need. He illustrates this with Dr. M. R. D. Hahn's statement about God's chemistry: in the physical realm something placed in blood comes out blood red or dark brown, but in God's chemistry, when something is placed in the blood of Christ, it comes out white as new fallen snow. Being justified is therefore a judicial act of God declaring us now just as if we've never sinned, like Adam before he fell, though not in the physical sense.